Conventionally, sponge iron used for manufacturing iron powder for powder metallurgy or chemical reactions has been manufactured by the following procedure: as shown in FIG. 1, a cylindrical body in which iron oxide 2 is placed between beds comprising a solid reducing agent 3 is packed into a cylindrical heat resistant container 1, called a saggar, and the resulting heat resistant container 1 is heated in a tunnel furnace to reduce the iron oxide 2. The sponge iron contains 90-97% by weight of Fe and is roughly crushed into a crude reduced iron powder including particles that can pass through a 90-mesh sieve, and the resulting crude reduced iron powder is provided for chemical reactions. For use in powder metallurgy, the crude reduced iron powder is reduced again in a non-oxidizing atmosphere such as hydrogen gas to finally produce a high-purity reduced iron powder containing at least 99.5% by weight of Fe. Generally, the iron oxide 2 includes iron ore and mill scale, and the solid reducing agent 3 includes a mixture of lime powder and carbonaceous powder such as coke powder. The process for manufacturing sponge iron and reduced iron powder is shown in FIG. 2.
Generally, reduced iron powder that is manufactured by crushing sponge iron or then reducing the crushed sponge iron consists of particles that are irregular shape and porous, so that the reduced iron powder has excellent compactability and sintering properties. Therefore, the reduced iron powder is suitable for raw materials for powder metallurgy and is used together with an atomized iron powder. The reduced iron is porous, has a specific surface area larger than that of the atomized iron powder, and readily reacts with oxygen. Therefore, the reduced iron powder is suitable for a wide range of applications, for example, chemically reactive materials such as deoxidizing agents and raw materials for portable body warmers.
Reduced iron powder that is manufactured by crushing sponge iron manufactured by reducing mill scale and then reducing the crushed sponge iron generally has high purity. However, there is a problem in that the reduced iron powder has an apparent density of 2.40-2.80 Mg/m3, which is relatively large, and has poor compactability.
Reduced iron powder that is manufactured by crushing sponge iron manufactured by reducing iron ore and then further reducing the crushed sponge iron has an apparent density of 1.70-2.50 Mg/m3, which is relatively small, and is used for porous bearing metal. However, there is a problem in that the reduced iron powder has a small compressibility. The iron oxide content in iron ore is small and iron ore usually contains gangue components such as SiO2 and Al2O3, wherein the SiO2 content is about 1-2% by mass and the Al2O3 content is about 0.2-1% by mass. Therefore, there is a risk that SiO2 and Al2O3 remaining in the reduced iron powder as inclusions causes the performance of the bearings to decrease.
In order to solve the above problems, a method for manufacturing reduced iron powder for powder metallurgy is disclosed in Japanese Unexamined Patent Application Publication No. 53-26710 as follows: a raw material including a mixture of powdered mill scale and 5-40% by mass of iron ore are roughly reduced into sponge iron, and the resulting sponge iron is crushed to remove impurities, is finally reduced, and is then crushed again into a reduced iron powder having an apparent density of 2.0-2.6 Mg/m3, wherein the reduced iron powder has excellent compactability and compressibility.
The reduced iron powder manufactured by the method disclosed in Japanese Unexamined Patent Application Publication No. 53-26710 has a small apparent density. However, there is a problem in that the purity is still low due to the use of iron ore, that is, inclusions remain in the reduced iron powder.